1. Field of the Invention
The present invention relates to an apparatus and method for controlling operation of a solenoid-operated valve, and more particularly to an apparatus and method for controlling operation of a solenoid-operated valve, which is actuated in a rated mode and a power-saving mode.
2. Description of the Related Art
Solenoid-operated valves are actuated by general-purpose output devices, such as a sequencer or the like. Generally, a large amount of electric power is required to move a movable element in a drive coil of a solenoid-operated valve in order to actuate the solenoid-operated valve, and after the movable element has moved, the movable element is held in position with a relatively small amount of electric power.
Based on such characteristics, there has been proposed a power-saving mode for controlling operation of solenoid-operated valves.
According to a first example of the power-saving mode, a rated voltage is applied to the drive coil of the solenoid-operated valve for a predetermined period of time based on a solenoid-operated valve drive command signal. During a remaining period of time of the solenoid-operated valve drive command signal, a holding voltage, which is lower than the rated voltage, is applied to the drive coil of the solenoid-operated valve following application of the rated voltage, for thereby actuating the solenoid-operated valve in the power-saving mode (see, for example, Japanese Laid-Open Patent Publication No. 3-213782).
According to a second example of the power-saving mode, a drive coil of a solenoid-operated valve is energized at a duty ratio of 100% for a predetermined period of time based on a solenoid-operated valve drive command signal under the control of a microprocessor, for moving a movable element in the drive coil. During a remaining period of time of the solenoid-operated valve drive command signal, following energization at a duty ratio of 100%, the drive coil is energized at a lower duty ratio in the power-saving mode in order to hold the movable element in the drive coil (see, for example, U.S. Pat. No. 6,164,323).
However, while the solenoid-operated valve is energized in the power-saving mode, the movable element is held under a weak force. When the solenoid-operated valve is subjected to a strong external shock or the like in the power-saving mode, the movable element tends to be displaced from the held position, causing the solenoid-operated valve to switch from one fluid passage to another fluid passage. At this time, a cylinder that has been actuated by a fluid flowing through the solenoid-operated valve may possibly operate unexpectedly.
To avoid the above drawback, the movable element of the solenoid-operated valve may be monitored by a sensor or the like. However, when the solenoid-operated valve is combined with such a sensor, the apparatus becomes relatively complex in structure and is costly to manufacture.